Increased Nutrient Acquisition Research Articles

Facilitation between intercropped species increases micronutrient acquisition and controls rust disease on maize

Context or ProblemGlobal food security is threatened by plant disease, and crop diversification often promotes productivity through reduced disease and facilitation for increased nutrient acquisition. However, whether such facilitation is a factor in disease resistance is unknown. Objective or Research QuestionInvestigate how crop diversity affects crop productivity and rust disease on maize, and whether competition or facilitation between crop species correspond with resistance. MethodsFive irrigated intercropping experiments with different fertilization and crop combinations were conducted at two sites for four years. Productivity (542 data points) and disease severity (27150 data points) of maize monoculture, maize - legume and maize - non-legume intercropping were compared. A meta-analysis of literature was performed to confirm the broader applicability of results from these field experiments. ResultsLegume-based intercropping increased the aboveground biomass of intercropped maize by 8% and grain yields by 10% in comparison to non-legume-based intercropping. Disease severity on maize intercropped with legumes was reduced by 45% and 48%, compared to monocultures of maize and maize intercropped with non-legumes, respectively. Moreover, as interactions among intercrops became more facilitative, the concentrations of zinc (Zn), copper (Cu) and iron (Fe) in maize increased, and these increases were highly correlated with decreasing disease severity. The global meta-analysis was consistent with our field experiments, as lower disease severity was associated with greater intensity of interspecific facilitation or with lower intensity of interspecific competition. Conclusions or SignificanceLower disease severity was closely related to enhanced acquisition of nutrients that can enhance the resistance to crop diseases, driven by stronger interspecific facilitative effects in intercropping systems. Facilitative effects on maize was increased by the identity of leguminous companion crop species, and was increased by sufficient irrigation, but reduced by applications of nitrogen and phosphorus fertilizers. ImplicationsOur findings identify a novel facilitative mechanism in general and advance the understanding of the facilitative mechanisms that underly disease control through crop diversification.

Dual functions of a novel effector in the plant and pathogen arms race

Ralstonia solanacearum is a soil-borne bacterium that causes bacterial wilt disease in over 250 plant species. It has been identified as one of the top ten most serious plant pathogenic bacteria globally, causing significant crop yield loss every year. Despite its large impact on agricultural economics, the molecular mechanisms underlying plant defense against Ralstonia infection and by which Ralstonia grows within the plant xylem remain largely unexplored. In a recent article, Ke et al. discovered a distinct pathogen effector, which acted as an immune elicitor in plants but also played dual roles in compromising plant immune activation and increasing nutrient acquisition from the host plants for pathogen propagation.

Changes in Chinese fir plantations root exudation strategies seasonally and as tree age

Root exudates play an important role in nutrient acquisition and defense of trees. However, there are few studies on the mechanism of productivity decline in late stage from the perspective of root exudates. Therefore, we collect root exudates in the summer and winter from 8-year-old (young), 18-year-old (middle-aged), and 49-year-old (overmature) trees in Chinese fir plantations to assess how their exudation rates and metabolite composition changed across these different life stages. Results showed that the rates and metabolite composition of root exudation varied significantly across plantation ages and between seasons. Specifically, the young Chinese fir trees increased exudation rates and produced more carbohydrates and quercetins, and overmature trees secreted more lipids and salicylic acids. This indicates that young Chinese fir plantations adopted root metabolic strategies which increased nutrient acquisition to satisfy the demands of rapid growth, but overmature plantations might produce more defensive metabolites to improve their defense capabilities. Additionally, in the winter, trees had slower root exudation rates and released a smaller proportion of organic acid, carbohydrate, and quercetin metabolites than in the summer, but a greater proportion of lipid and salicylic acid metabolites. We also found that soil microbial factors (microbial community composition and diversity) had a significant effect on the composition of primary and secondary metabolites in the winter. Overall, as plantation age increased, Chinese fir root metabolic strategies related to nutrient acquisition decrease, while those related to defense increase. These findings provide a new perspective for our understanding of the mechanisms that lead to productivity declines of Chinese fir plantations in later growth stages.

Plant microbe mediated enhancement in growth and yield of canola (Brassica napus L.) plant through auxin production and increased nutrient acquisition

Plant microbe mediated enhancement in growth and yield of canola (Brassica napus L.) plant through auxin production and increased nutrient acquisition

Transcriptomic and Metabolomic Analysis of a Fusidic Acid-Selected fusA Mutant of Staphylococcus aureus.

Physiological experimentation, transcriptomics, and metabolomics were engaged to compare a fusidic acid-resistant Staphylococcus aureus mutant SH10001st-2 to its parent strain SH1000. SH10001st-2 harbored a mutation (H457Y) in the gene fusA which encodes the fusidic acid target, elongation factor G, as well as mutations in a putative phage gene of unknown function. SH10001st-2 grew slower than SH1000 at three temperatures and had reduced coagulase activity, two indicators of the fitness penalty reported for fusA-mediated fusidic acid- resistance in the absence of compensatory mutations. Despite the difference in growth rates, the levels of O2 consumption and CO2 production were comparable. Transcriptomic profiling revealed 326 genes were upregulated and 287 were downregulated in SH10001st-2 compared to SH1000. Cell envelope and transport and binding protein genes were the predominant functional categories of both upregulated and downregulated genes in SH10001st-2. Genes of virulence regulators, notably the agr and kdp systems, were highly upregulated as were genes encoding capsule production. Contrary to what is expected of mid-exponential phase cells, genes encoding secreted virulence factors were generally upregulated while those for adhesion-associated virulence factors were downregulated in SH10001st-2. Metabolomic analysis showed an overall increase in metabolite pools in SH10001st-2 compared to SH1000, mostly for amino acids and sugars. Slowed growth and metabolite accumulation may be byproducts of fusA mutation-mediated protein synthesis impairment, but the overall results indicate that SH10001st-2 is compensating for the H457Y fitness penalty by repurposing its virulence machinery, in conjunction with increasing metabolite uptake capacity, in order to increase nutrient acquisition.

Effects of Soybean Lecithin on Growth Performance, Intestine Morphology, and Liver Tissue Metabolism in Rock Bream (Oplegnathus fasciatus) Larvae

Investigations have demonstrated a strong and positive association between dietary intact phospholipid (PL) inclusion and aquatic larval growth, nevertheless, the precise molecular mechanism underlying PL inclusion on growth performance has not been well elucidated. This study aimed to investigate the effects of dietary soybean lecithin (SL) inclusion on growth performance, liver metabolism, resistance to hypoxia stress, and potential molecular mechanisms in rock bream (Oplegnathus fasciatus) larvae. Four types of equal-protein and equal-lipid content microdiets (MDs) were formulated with graded levels of SL to achieve phospholipid levels of (PLs, dry matter) 3.84% (SL0), 6.71% (SL4), 9.38% (SL8), and 12.21% (SL12). Rock bream larvae (25 days post-hatching) were fed the respective MDs for 30 days with three replicates. We found that dietary SL inclusion promoted growth performance, survival rate, and stress resistance to hypoxia stress. The increased dietary SL inclusion improved intestinal structure, as shown by the increased perimeter ratio, muscular thickness, and mucosal fold height of the mid-intestinal tissue. Moreover, a high SL inclusion diet (SL12) increased the activity of the key lipolysis-related enzyme (lipase [LP]) in liver tissue but decreased the activity of amino acid catabolism-related enzymes (aspartate aminotransferase [AST] and alanine aminotransferase [ALT]). RNA sequencing results in liver tissue revealed that the SL12 diet increased the transcriptional level of fatty acid activation-related genes (acsl6 and acsbg2), phospholipid catabolism-related genes (acat2, lpin2, and crls), and amino acid synthesis-related genes (gs, csb, aldh18a1, and oct), but decreased the expression of amino acid catabolism-related gene gprt2. Notably, the SL12 diet significantly increased the expression of ribosome biogenesis-related genes (pes1, nop56, nop58, and rpf2) in liver tissue. The ribosome protein-related pathways were the most enriched pathways mapped in the GO database. Collectively, this study demonstrated the necessity of dietary SL for survival, growth performance, promotion of mid-intestinal morphology, and hypoxia stress during the rock bream larval stage. The SL-induced growth performance promotion was likely attributed to increasing nutrient acquisition by intestinal morphology improvement and to increasing SL catabolism and thereby sparing amino acids for protein synthesis.

Bacillus atrophaeus WZYH01 and Planococcus soli WZYH02 Improve Salt Tolerance of Maize (Zea mays L.) in Saline Soil.

With the increasing shortage of land resources and people’s attention to the ecological environment, the application of microbial fertilizer with natural soil microorganisms as the main component has attracted increasing attention in saline agriculture. In this study, two salt-tolerant strains, YL07 (Bacillus atrophaeus) and YL10 (Planococcus soli), were isolated from maize (Zea mays L.) rhizosphere soil with a saturated conductivity (ECe) of 6.13 dS/m and pH of 8.32 (Xinjiang, China). The effects of B. atrophaeus WZYH01 (YL07) and Planococcus soli WZYH02 (YL10) on the growth and development of maize (Zea mays L.) under salt stress (ECe = 5.9 dS/m) were further studied. The results showed that compared with uninoculation, inoculation with B. atrophaeus WZYH01 and Planococcus soli WZYH02 significantly improved maize growth performance, biomass yield, and antioxidant levels under salt stress, and the effect of Planococcus soli WZYH02 was more prominent than the effect of B. atrophaeus WZYH01. Moreover, inoculation with B. atrophaeus WZYH01 and Planococcus soli WZYH02 protected maize from salt stress by regulating plant hormone [IAA and abscisic acid (ABA)] levels and increasing nutrient acquisition. In addition, the tested strains were most efficient for maize growth and health, increasing the content of K+ accompanied by an effective decrease in Na+ in maize tissues. The transcription levels of salt tolerance genes (ZMNHX1, ZMNHX2, ZMHKT, ZMWRKY58, and ZMDREB2A) in inoculated maize were also dramatically higher than the transcription levels of the specified salt tolerance genes in uninoculated maize. In conclusion, B. atrophaeus WZYH01 and Planococcus soli WZYH02 can alleviate the harmful effects of salt stress on crop growth, thereby promoting sustainable agricultural development.

Isolation, Biochemical Characterization and Antagonistic Study of Pseudomonas Fluorescens in Soil Collected from Different Locations in Kerala

Plant Growth Promoting Rhizobacteria (PGPR) is bacteria that populates roots and enhance the growth by a variety of mechanisms. Generally, PGPR promote plant growth directly by either increasing nutrient acquisition for example nitrogen, phosphorus, potassium and essential minerals, or modulating plant hormone levels, or by indirectly decreasing the inhibitory effects of pathogens on plant growth and development as biocontrol agents. In this study, an attempt was made to collect rhizospheric soil samples with native Pseudomonas population from different locations of Kerala state and to identify the native Pseudomonas fluorescens strains, a potent biocontrol agent in controlling pest, insects, nematodes and various pathogenic microbes as well as PGPR in the rhizosphere under UV light and further characterize them morphologically and culturally. Utmost P. fluorescens strains showed positive PGPR exertion. The study showed that Pseudomonas is an effective factory growth promoting bacterium. In the present study, soil samples were collected from different locales in Kerala (Trivandrum, Kollam and Kottayam) in order to insulate the soil microbes and also checking its negative parcels with other microbes. The observed colonies were identified as Pseudomonas fluorescens by checking the green fluorescence emitted by the colonies when exposed to UV light. These fluorescent colonies were selected and confirmed as Pseudomonas fluorescens using biochemical tests like MR test, Citrate utilisation test and Catalase test. The Methyl red (MR) test was used to identify the glucose utilizing property of Pseudomonas fluorescens. A change from yellow to cherry red colour indicates positive result. The Citrate Utilisation test was used to identify the ability of Pseudomonas fluorescens to utilize citrate as its sole carbon and energy source thereby confirming that the isolated colony is of the particular species. Catalase test was used to identify the ability of catalase containing Pseudomonas fluorescens to convert Hydrogen peroxide into Water and Oxygen. Presence of thick effervescence was noted confirming the catalase utilising ability of Pseudomonas fluorescens. Coagulase test is used in order to differentiate Staphylococcus aureus that is positive which produce the enzyme coagulase, from S. epidermis and S. saprophyticus negative one which do not produce coagulase.

Mycorrhizal Symbioses Enhance Competitive Weed Growth in Biochar and Nutrient-Amended Soils

Velvetleaf (Abutilon theophrasti) is a highly competitive weed in agroecosystems that is well-studied for its efficient nitrogen (N) acquisition, yet research on its phosphorus (P) uptake is lacking. One pathway may be through symbioses with arbuscular mycorrhizal fungi (AMF) which increase nutrient acquisition. These AMF benefits can be further enhanced by soil amendment with biochar, although effects may vary with different biochar production characteristics. We implemented a fully factorial nutrient and biochar addition experiment in a greenhouse for six months to determine how AMF nutrient uptake impacts plant growth and how these effects vary between two biochar types. We measured total above- and belowground biomass, plant tissue concentration (N and P), AMF colonization and activity rates, and soil media N and P availability. Overall, we observed few statistically significant results, however AMF N uptake may have been more beneficial to velvetleaf than AMF P uptake as evidenced by increased biomass and tissue N concentrations in treatments where N was only accessible by AMF. Additionally, by maintaining root to shoot ratios biochar may have provided plants with N and P (through sorption of nutrients to surfaces or its inherent properties) when nutrients were more difficult to access. We also found variable plant responses across the two biochar types used. Understanding how nutrient and biochar additions can influence weed competition is important for anticipating potential undesirable consequences of novel soil amendments such as biochar.

A FRET-Based Fluorogenic Trehalose Dimycolate Analogue for Probing Mycomembrane-Remodeling Enzymes of Mycobacteria.

The mycobacterial outer membrane, or mycomembrane, is essential for the viability and virulence of Mycobacterium tuberculosis and related pathogens. The mycomembrane is a dynamic structure, whose chemical composition and biophysical properties can change during stress to give an advantage to the bacterium. However, the mechanisms that govern mycomembrane remodeling and their significance to mycobacterial pathogenesis are still not well characterized. Recent studies have shown that trehalose dimycolate (TDM), a major glycolipid of the mycomembrane, is broken down by the mycobacteria-specific enzyme TDM hydrolase (Tdmh) in response to nutrient deprivation, a process which appears to modulate the mycomembrane to increase nutrient acquisition, but at the expense of stress tolerance. Tdmh activity thus balances the growth of M. tuberculosis during infection in a manner that is contingent upon host immunity. Current methods to probe Tdmh activity are limited, impeding the development of inhibitors and the investigation of the role of Tdmh in bacterial growth and persistence. Here, we describe the synthesis and evaluation of FRET-TDM, which is a fluorescence-quenched analogue of TDM that is designed to fluoresce upon hydrolysis by Tdmh and potentially other trehalose ester-degrading hydrolases involved in mycomembrane remodeling. We found that FRET-TDM was efficiently activated in vitro by recombinant Tdmh, generating a 100-fold increase in fluorescence. FRET-TDM was also efficiently activated in the presence of whole cells of Mycobacterium smegmatis and M. tuberculosis, but the observed signal was predominantly Tdmh-independent, suggesting that physiological levels of Tdmh are low and that other mycobacterial enzymes also hydrolyze the probe. The latter notion was confirmed by employing a native protein gel-based fluorescence assay to profile FRET-TDM-activating enzymes from M. smegmatis lysates. On the other hand, FRET-TDM was capable of detecting the activity of Tdmh in cells when it was overexpressed. Together, our data demonstrate that FRET-TDM is a convenient and sensitive in vitro probe of Tdmh activity, which will be beneficial for Tdmh enzymatic characterization and inhibitor screening. In more complex samples, for example, live cells or cell lysates, FRET-TDM can serve as a tool to probe Tdmh activity at elevated enzyme levels, and it may facilitate the identification and characterization of related hydrolases that are involved in mycomembrane remodeling. Our study also provides insights as to how the structure of FRET-TDM or related fluorogenic probes can be optimized to achieve improved specificity and sensitivity for detecting mycobacteria.

P and N deficiency change the relative abundance and function of rhizosphere microorganisms during cluster root development of white lupin (Lupinus albus L.)

ABSTRACTWe studied microbe-plant interactions of white lupin, a cluster root-forming plant, under low P and N conditions to examine increased nutrient acquisition by plants either by a shift to a more specialized microbial community or changes in microbial enzyme production. White lupin plants were grown in rhizoboxes filled with either P- or N-deficient soil; fertilized soil was used as control. After cultivation of plants in a glasshouse for 41 d, plant growth (shoot and roots) and P and N accumulation in shoots were measured. Microbial functions were analyzed by P- and N-cycling enzymes. The microbial community structure was estimated by fingerprinting (denaturing gradient gel electrophoresis) and sequencing techniques. P deficiency induced the released citrate and acid phosphomonoesterases from cluster roots and stimulated the production of microbe-derived alkaline phosphomonoesterase in the rhizosphere. P deficiency decreased microbial diversity in the cluster root rhizosphere. Increased relative abundance of Burkholderiales in the rhizosphere of P deficient plants might be responsible for the degradation of different organic P fractions such as phytates. N deficiency induced an increase of the number of nodules and P concentration in shoot as well as roots of white lupin. We clarified that high release of citrate from cluster roots might be the preferred mechanisms to meet the P demand of nodulated plants under N deficiency. In addition, the high abundance of Rhizobiales and Rhodospirillales in the rhizosphere of cluster roots showed that the importance of N-fixing microorganisms under N deficiency. The contribution of rhizosphere microorganisms due to similar activities of N-cycling enzymes under the two different N treatments is less important for N nutrition of plants. Further understanding of the regulation of cluster roots under N-deficiency will provide new information on the interactions between P and N nutrition.

Organic amendments increase crop yields by improving microbe-mediated soil functioning of agroecosystems: A meta-analysis

Although numerous studies suggest that organic amendments are better at maintaining soil fertility and crop production than mineral-only fertilization, it is unclear if this occurs in different agricultural systems on a global scale. Here we report a comprehensive meta-analysis of 690 independent experiments comparing the performance of organic amendments and mineral-only fertilization on crop yields, the soil organic carbon (SOC) and total nitrogen (TN) contents, soil nutrient dynamics and biological properties. Our analysis shows that organic amendments increased crop yields on average of 27% than mineral-only fertilization. Farmyard manure (FYM) had the highest effect (49% increase) and this was especially clear in wheat croplands (40% increase). Organic amendment increased the amount of SOC (38%), TN (20%), microbial biomass carbon (MBC; 51%) and microbial biomass nitrogen (MBN; 24%) than mineral-only fertilization. Organic amendments also increased the soil microbiome enzyme activity in terms of soil hydrolytic C acquisition (C-acq; 39%), N acquisition (N-acq; 22%), P acquisition (P-acq; 48%) and oxidative decomposition (OX; 58%). Increased nutrient acquisition and oxidative decomposition could explain the positive effects of organic amendment on crop yields. These observed patterns were consistent for most organic amendments and cropping systems in diverse regions of the world. In summary, our analysis suggests that organic amendments can improve microbe-mediated soil ecosystem functioning, long-term soil fertility and crop productivity, relative to mineral fertilization, on a global scale.

Trichoderma asperellum T42 Reprograms Tobacco for Enhanced Nitrogen Utilization Efficiency and Plant Growth When Fed with N Nutrients.

Trichoderma spp., are saprophytic fungi that can improve plant growth through increased nutrient acquisition and change in the root architecture. In the present study, we demonstrate that Trichoderma asperellum T42 mediate enhancement in host biomass, total nitrogen content, nitric oxide (NO) production and cytosolic Ca2+ accumulation in tobacco. T42 inoculation enhanced lateral root, root hair length, root hair density and root/shoot dry mass in tobacco under deprived nutrients condition. Interestingly, these growth attributes were further elevated in presence of T42 and supplementation of NO3- and NH4+ nutrients to tobacco at 40 and 70 days, particularly in NO3- supplementation, whereas no significant increment was observed in nia30 mutant. In addition, NO production was more in tobacco roots in T42 inoculated plants fed with NO3- nutrient confirming NO generation was dependent on NR pathway. NO3- dependent NO production contributed to increase in lateral root initiation, Ca2+ accumulation and activities of nitrate transporters (NRTs) in tobacco. Higher activities of several NRT genes in response to T42 and N nutrients and suppression of ammonium transporter (AMT1) suggested that induction of high affinity NRTs help NO3- acquisition through roots of tobacco. Among the NRTs NRT2.1 and NRT2.2 were more up-regulated compared to the other NRTs. Addition of sodium nitroprusside (SNP), relative to those supplied with NO3-/NH4+ nutrition and T42 treated plants singly, and with application of NO inhibitor, cPTIO, confirmed the altered NO fluorescence intensity in tobacco roots. Our findings suggest that T42 promoted plant growth significantly ant N content in the tobacco plants grown under N nutrients, notably higher in NO3-, providing insight of the strategy for not only tobacco but probably for other crops as well to adapt to fluctuating nitrate availability in soil.

Root symbionts: Powerful drivers of plant above‐ and belowground indirect defenses

Soil microbial mutualists of plants, including mycorrhizal fungi, non-mycorrhizal fungi and plant growth promoting rhizobacteria, have been typically characterized for increasing nutrient acquisition and plant growth. More recently, soil microbes have also been shown to increase direct plant defense against above- and belowground herbivores. Plants, however, do not only rely on direct defenses when attacked, but they can also recruit pest antagonists such as predators and parasitoids, both above and belowground, mainly via the release of volatile organic compounds (i.e., indirect defenses). In this review, we illustrate the main features and effects of soil microbial mutualists of plants on plant indirect defenses and discuss possible applications within the framework of sustainable crop protection against root- and shoot-feeding arthropod pests. We indicate the main knowledge gaps and the future challenges to be addressed in the study and application of these multifaceted interactions.

Influence of arbuscular mycorrhizal colonization on whole-plant respiration and thermal acclimation of tropical tree seedlings.

Symbiotic arbuscular mycorrhizal fungi (AMF) are ubiquitous in tropical forests. AMF play a role in the forest carbon cycle because they can increase nutrient acquisition and biomass of host plants, but also incur a carbon cost to the plant. Through their interactions with their host plants they have the potential to affect how plants respond to environmental perturbation such as global warming. Our objective was to experimentally determine how plant respiration rates and responses to warmer environment are affected by AMF colonization in seedlings of five tropical tree species at the whole plant level. We evaluated the interaction between AMF colonization and temperature on plant respiration against four possible outcomes; acclimation does or does not occur regardless of AMF, or AMF can increase or decrease respiratory acclimation. Seedlings were inoculated with AMF spores or sterilized inoculum and grown at ambient or elevated nighttime temperature. We measured whole plant and belowground respiration rates, as well as plant growth and biomass allocation. There was an overall increase in whole plant, root, and shoot respiration rate with AMF colonization, whereas temperature acclimation varied among species, showing support for three of the four possible responses. The influence of AMF colonization on growth and allocation also varied among plant species. This study shows that the effect of AMF colonization on acclimation differs among plant species. Given the cosmopolitan nature of AMF and the importance of plant acclimation for predicting climate feedbacks a better understanding of the patterns and mechanisms of acclimation is essential for improving predictions of how climate warming may influence vegetation feedbacks.

Intercropping maintains soil fertility in terms of chemical properties and enzyme activities on a timescale of one decade

Overyielding (i.e., mixtures of crops yielding higher than expected when compared with monocultures) and increased nutrient acquisition have been found in many intercropping systems. However, there are very few published studies on long-term changes in soil chemical and biological properties in intercropping systems compared to sole cropping. A field experiment was established in 2003 in Gansu province, northwest China. The treatments comprised three intercropping systems (either continuous or rotational wheat/maize, wheat/faba bean, maize/faba bean intercropping), rotational cropping (wheat-maize, wheat-faba bean, faba bean-maize, and wheat-maize-faba bean rotations), and monocropping (sole wheat, faba bean and maize) systems. In 2011 (ninth year of the experiment) and 2012 (tenth year) the yields and some soil chemical and biological properties were examined after all crop species were harvested. There was overyielding by 6.6 % and 32.4 % in wheat/maize intercropping in 2011 and 2012, respectively. Faba bean/maize intercropping was enhanced by 34.7 % and 28.6 %, respectively but not wheat/faba bean intercropping. Soil organic matter, total nitrogen, Olsen P, exchangeable K and cation exchange capacity in all intercropping systems did not differ from the monocultures except for soil pH in wheat/maize and faba bean/maize intercropping in 2011 and soil exchangeable K and cation exchange capacity (CEC) in 2012. Soil pH in wheat/maize and faba bean/maize intercropping was significantly reduced by 3.2 % and 1.9 %, respectively. Soil exchangeable K in wheat/maize, faba bean/maize and wheat/faba bean intercropping declined markedly by 15 %, 21.7 % and 12.1 %, respectively. Soil cation exchange capacity in wheat/maize, faba bean/maize and wheat/faba bean intercropping was notably lower than the corresponding monocultures by 17.5 %, 23.3 % and 18.3 %, respectively. Soil enzyme activities after 9 and 10 years of intercropping differed little from monocultures or rotations. The results indicate that intercropping overyielded compared with monocropping or rotational cropping and also maintained the stability of most of the soil chemical and enzyme activities relative to rotations and monocropping in the relatively fertile soil studied.

Organic matter dynamics and microbial activity during decomposition of forest floor under two native neotropical oak species in a temperate deciduous forest in Mexico

Litter decomposition, which represents the main pathway of nutrient return to soil, affects the fertility of forest fragments. The biochemical composition of litter and the way that it decomposes are determined by the physiological characteristics of the plant species from which the material is derived. The main physiological process affecting nutrient concentrations in litter is the resorption of foliar nutrients. In this study, we determined organic matter composition, nutrient dynamics and microbial activity in samples of forest floor litter collected over a two year period under two native oak species, Quercus castanea (Qc) and Quercus deserticola (Qd), growing in a semi-natural deciduous forest in central Mexico. These coexisting oak species differ in relation to nutrient resorption efficiency, and their spatial distribution is vulnerable to climate change. The Nuclear Magnetic Resonance (13C-NMR) spectra revealed a higher O-alkyl:aromatic C ratio and more decomposable lignin structure (based on lignin subunit ratios) in the Qd litterfall than in the Qc litterfall (3.7 and 3.6, respectively), in concordance with results obtained by Fourier Transform Infrared (FTIR) Spectroscopy and Differential Scanning Calorimetry (DSC). Consequently, the Qd litter had a higher proportion of thermolabile compounds. In the same way, the nutrient concentrations were very different in each species, as a result of the different patterns of foliar nutrient resorption: both nutrient concentrations and microbial activity were higher in the Qd litter than in the Qc litter. Moreover, the specific enzymatic activity (SEA) of the microbial community was higher in the Qc litter, suggesting that these microorganisms required investment of more energy to increase nutrient acquisition rather than to increase microbial growth. The results suggest that the chemical composition and chemical quality of litterfall, resulting from foliar resorption, strongly affect forest floor microbial activity. Consequently, the physiological footprint of both tree species is a key factor in decomposition processes and soil fertility.

Arbuscular mycorrhizal fungi and aphids interact by changing host plant quality and volatile emission

Summary Most plants interact with both arbuscular mycorrhizal (AM) fungi, which increase nutrient acquisition, and herbivores such as aphids, which drain nutrients from plants. Both AM fungi and aphids can affect plant metabolic pathways and may influence each other by altering the condition of the shared host plant. This study tests simultaneously the effects of AM fungi on interactions with aphids (bottom‐up effects) and the effects of aphids on interactions with AM fungi (top‐down effects). We hypothesized that: (i) attractiveness of plants to aphids is regulated by induced changes in production of plant volatile organic compounds (VOCs) triggered by AM fungi or aphids; (ii) aphids reduce AM fungal colonization; and (iii) AM fungal colonization affects aphid development. Broad beans were exposed to AM fungi, aphids and a combination of both. To test for the strength of bottom‐up and top‐down effects, separate treatments enabled establishment of mycorrhizas either before or after aphids were added to plants. VOCs produced by plants were used to (i) test their attractiveness to aphids and (ii) identify the semiochemicals causing attraction. We also measured plant growth and nutrition, AM fungal colonization and aphid reproduction. AM fungi increased the attractiveness of plants to aphids, and this effect tended to prevail even for aphid‐infested plants. However, both attractiveness and aphid population growth depended on the timing of AM fungal inoculation. AM fungi suppressed emission of the sesquiterpenes (E)‐caryophyllene and (E)‐β‐farnesene, and aphid attractiveness to VOCs was negatively associated with the proportion of sesquiterpenes in the sample. Emission of (Z)‐3‐hexenyl acetate, naphthalene and (R)‐germacrene D was regulated by an interaction between aphids and AM fungi. Aphids had a negative effect on mycorrhizal colonization, plant biomass and nutrition. Our data show that below‐ and above‐ground organisms can interact by altering the quality of their shared host plant even though there is no direct contact between them. Plant interactions with herbivores and AM fungi operate in both directions: AM fungi have a key bottom‐up role in insect host location by increasing the attractiveness of plant VOCs to aphids, whereas aphids inhibit formation of AM symbioses.

A root is a root is a root? Water uptake rates of Citrus root orders

Knowledge about the physiological function of root orders is scant. In this study, a system to monitor the water flux among root orders was developed using miniaturized chambers. Different root orders of 4-year-old Citrus volkameriana trees were analysed with respect to root morphology and water flux. The eight root orders showed a broad overlap in diameter, but differences in tissue densities and specific root area (SRA) were clearly distinguishable. Thirty per cent of the root branch biomass but 50% of the surface area (SA) was possessed by the first root order, while the fifth accounted for 5% of the SA (20% biomass). The root order was identified as a determinant of water flux. First-order roots showed a significantly higher rate of water uptake than the second and third root orders, whereas the fourth and fifth root orders showed water excess. The water excess suggested the occurrence of hydraulic redistribution (HR) as a result of differences in osmotic potentials. We suggest that plants may utilize hydraulic redistribution to prevent coarse root desiccation and/or to increase nutrient acquisition. Our study showed that the novel 'miniature depletion chamber' method enabled direct measurement of water fluxes per root order and can be a major tool for future studies on root order traits.

The effects of elevated CO2 on root respiration rates of two Mojave Desert shrubs

AbstractAlthough desert ecosystems are predicted to be the most responsive to elevated CO2, low nutrient availability may limit increases in productivity and cause plants in deserts to allocate more resources to root biomass or activity for increased nutrient acquisition. We measured root respiration of two Mojave Desert shrubs, Ambrosia dumosa and Larrea tridentata, grown under ambient (∼375 ppm) and elevated (∼517 ppm) CO2 concentrations at the Nevada Desert FACE Facility (NDFF) over five growing seasons. In addition, we grew L. tridentata seedlings in a greenhouse with similar CO2 treatments to determine responses of primary and lateral roots to an increase in CO2. In both field and greenhouse studies, root respiration was not significantly affected by elevated CO2. However, respiration of A. dumosa roots <1 month old was significantly greater than respiration of A. dumosa roots between 1 and 4 months old. For both shrub species, respiration rates of very fine (<1.0 mm diameter) roots were significantly greater than those of fine (1–2 mm diameter) roots, and root respiration decreased as soil water decreased. Because specific root length was not significantly affected by CO2 and because field minirhizotron measurements of root production were not significantly different, we infer that root growth at the NDFF has not increased with elevated CO2. Furthermore, other studies at the NDFF have shown increased nutrient availability under elevated CO2, which reduces the need for roots to increase scavenging for nutrients. Thus, we conclude that A. dumosa and L. tridentata root systems have not increased in size or activity, and increased shoot production observed under elevated CO2 for these species does not appear to be constrained by the plant's root growth or activity.